The Ovule and the Embryo Sac.

The plant life cycle is characterized by the alternation of generations between a diploid sporophyte and a haploid gametophyte. The sporophyte functions to produce spores, which then develop into gametophytes. The differentiated gametophytes in turn produce either the male gametes (sperm) or female gametes (egg cells). In contrast to lower plant species, in which the gametophyte is the dominant, free-living generation (see Cove and Knight, 1993, this issue), gametophytes of angiosperms are smaller and less complex than the sporophyte and are formed within specialized organs of the flower. The male gametophyte (pollen or microgametophyte) develops within the anther, whereas the female gametophyte (embryo sac or megagametophyte) is a product of the ovule. Sexual reproduction requires the delivery of the sperm nuclei, via the pollen, to the embryo sac, where fertilization occurs and the new diploid sporophyte is formed (see Dumas and Mogensen, 1993, this issue). These unique features of the angiosperm reproductive process illustrate one of the most fundamental problems in plant biology. That is, how does the transition from sporophytic to gametophytic development occur within the context of the flower? How do the gametophytes interact with each other and the maternal plant to produce a seed? A wealth of information has been obtained regarding sexual reproduction in plants by genetic and molecular analyses of pollen development (Mascarenhas, 1990,1993, this issue) and pollen-stigmaktyle interactions (Cornish et al., 1988; Nasrallah and Nasrallah, 1989, 1993, this issue; Newbigin et al., 1993, this issue). By comparison, less is known about the regulation of female gametophyte development and the relationship of the embryo sac to the ovule before and after fertilization. The ovule is the source of the megagametophyte and the progenitor of the seed. Specification of the megasporocyte, production of a functional megaspore (megasporogenesis), formation of the embryo sac (megagametogenesis), and embryogenesis all occur within the ovule. Because embryo sacs, unlike pollen, maintain physical contact with the parent sporophyte throughout their development, this association of the female gametophyte and the sporophyte provides an opportunity to examine interactions between cells, tissues, and genomes.